Recently, various optical devices have been proposed. For example, an actively absorptive polarizer to which a guest-host type liquid crystal cell is applicable, a first statically reflective polarizer, an active polarization rotor, and an electronically light-controllable optical device comprising the first statically reflective polarizer have been proposed. In addition, a reflectance variable system or transmittance variable system comprising a variable polarizer to which a gust-host type liquid crystal cell is applicable has also been proposed. The optical device is required to improve the transmittance or transparency on one hand and to implement black display and improve the display quality on the other hand.